In recent years, a technique has been developed in which chromatic aberration is corrected using a diffractive optical element in an imaging optical system, and optical materials having a low refractive index and a high dispersion which are favorable for use in it attract notice. For example, in a close-contact double-layer type diffractive optical element consisting of two layers having different optical characteristics, as proposed in Japanese Patent Laid-open Publication No. H9-127321, an optical material having a low refractive index and a high dispersion is used in one of layers constituting the element.
However, it is known that optical materials have a positive correlation between the refractive index and the dispersion. Materials having a low refractive index has a small dispersion, and conversely materials having a high refractive index has a large dispersion. Hence, optical materials having a high dispersion though having a low refractive index are very limited. In fact, materials disclosed in the above publication are almost inorganic materials, and just only polycarbonates are disclosed as organic materials.
Accordingly, inorganic matter, in particular, a glass is usually used as an optical material in such a close-contact double-layer type diffractive optical element. For example, in Japanese Patent Laid-open Publication No. 2001-235608, a method is disclosed in which a close-contact double-layer type diffractive optical element is produced using a low-melting glass.
However, in general, inorganic materials are inferior to organic materials in respect of workability and moldability, and also it is not easy to bring inorganic optical materials into perfect close contact with each other. In addition, glass materials are necessarily limited and moreover required to have heat resistance because they are molded at a high temperature. Hence, materials for molds are also limited. Also, because of a high viscosity of glass materials at the time of molding, it has been difficult to materialize sufficient transfer properties.